datetime: interval initialization via pairs

Fixed incorrect interval initialization for case of `()` used
while it was correct for `{}` arguments.

NO_DOC=bugfix
NO_CHANGELOG=bugfix
NO_TEST=bugfix

```
tarantool> itv = require('datetime').interval.new{}
---
...

tarantool> itv
---
- 0 seconds
...

tarantool> itv = require('datetime').interval.new()
---
...

tarantool> itv
---
- +0.000000001 seconds
...
```

(cherry picked from commit d183ae6d)